U.S. patent number 5,017,551 [Application Number 07/330,409] was granted by the patent office on 1991-05-21 for barrier layer containing conductive articles.
This patent grant is currently assigned to Eastman Kodak Company. Invention is credited to John A. Agostinelli, Mark Lelental, Jose M. Mir, Ralph A. Nicholas, III, Gustavo R. Paz-Pujalt.
United States Patent |
5,017,551 |
Agostinelli , et
al. |
May 21, 1991 |
Barrier layer containing conductive articles
Abstract
A circuit element is disclosed comprised of a substrate and an
electrically conductive layer located on the substrate. The
electrically conductive layer is comprised of a crystalline rare
earth alkaline earth copper oxide. The substrate is formed of a
material which increases the electrical resistance of the
conductive layer when in contact with the rare earth alkaline earth
copper oxide during crystallization of the latter to an
electrically conductive form. A barrier layer is interposed between
the electrically conductive layer and the substrate. The barrier
layer contains magnesium, a group IVA metal, or a platinum group
metal, either in an elemental state or in the form of an oxide or
silicide. The circuit element is produced by first forming the
barrier layer on the substrate followed by coating conductor
precursor metal-ligand compounds of each of rare earth, alkaline
earth, and copper containing at least one thermally volatilizable
ligand and heating the precursor metal-ligand compounds in the
presence of oxygen to produce a crystalline rare earth alkaline
earth copper oxide electrically conductive layer.
Inventors: |
Agostinelli; John A.
(Rochester, NY), Mir; Jose M. (Webster, NY), Paz-Pujalt;
Gustavo R. (Rochester, NY), Lelental; Mark (Rochester,
NY), Nicholas, III; Ralph A. (Rochester, NY) |
Assignee: |
Eastman Kodak Company
(Rochester, NY)
|
Family
ID: |
27366935 |
Appl.
No.: |
07/330,409 |
Filed: |
March 30, 1989 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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85047 |
Aug 13, 1987 |
|
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46593 |
May 4, 1987 |
4880770 |
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Current U.S.
Class: |
505/235; 505/236;
505/237; 257/E39.018; 428/426; 428/433; 428/700; 428/930; 505/702;
505/704; 428/432; 428/688; 428/901; 505/701; 505/703 |
Current CPC
Class: |
C03C
17/23 (20130101); H01L 39/2425 (20130101); C04B
35/4504 (20130101); C04B 41/5025 (20130101); C04B
41/87 (20130101); C23C 2/26 (20130101); C23C
26/00 (20130101); H01L 39/143 (20130101); H01L
39/2461 (20130101); H01L 39/248 (20130101); Y10S
428/93 (20130101); Y10S 505/704 (20130101); Y10S
428/901 (20130101); Y10S 505/701 (20130101); Y10S
505/702 (20130101); Y10S 505/703 (20130101); C03C
2217/23 (20130101) |
Current International
Class: |
C04B
35/45 (20060101); C04B 41/50 (20060101); C04B
41/87 (20060101); C04B 41/45 (20060101); B32B
18/00 (20060101); C03C 17/23 (20060101); C04B
35/01 (20060101); C23C 26/00 (20060101); C23C
2/26 (20060101); H01L 39/14 (20060101); H01L
39/24 (20060101); B32B 003/02 () |
Field of
Search: |
;505/1,701-704
;428/209,426,432,433,688,700,901,930 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Applied Phys. Letts. 55(5), 7-31-89, Chu et al., pp. 492-494. .
Applied Phys. Letts. 53(25), 12-19-88, Mogro-Campero et al., pp.
2566-2568. .
J. G. Bednorz and K. A. Muller, "Possible High T.sub.c
Superconductivity in the Ba-La-Cu-O System", Z. Phys. B.--Condensed
Matter, vol. 64, pp. 189-193 (1986). .
C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, and Y. Q.
Wang, "Evidence for Superconductivity above 40 K in the La-Ba-Cu-O
Compound System", Physical Review Letters, vol. 58, No. 4, pp.
405-407, Jan. 1987. .
C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, and Z. J. Huang,
"Superconductivity at 52.5 K in the Lanthanum-Barium-Copper-Oxide
System", Science Reports, vol. 235, pp. 567-569, Jan. 1987. .
R. J. Cava, R. B. vanDover, B. Batlog, and E. A. Rietman, "Bulk
Superconductivity at 36 K in La.sub.1.8 Sr.sub.0.2 CuO.sub.4 ",
Physical Review Letters, vol. 58, No. 4, pp. 408-410, Jan. 1987.
.
J. M. Tarascon, L. H. Greene, W. R. McKinnon, G. W. Hull, and T. H.
Geballe, "Superconductivity at 40 K in the Oxygen-Defect
Perovskites La.sub.2-x Sr.sub.x CuO.sub.4-y ", Science Reports,
vol. 235, pp. 1373-1376, Mar. 13, 1987. .
M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng, L.
Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, "Superconductivity at
93 K in a New Mixed-Phase Y-Ba-Cu-O Compound System at Ambient
Pressure", Physical Review Letters, vol. 58, No. 9, pp. 908-910,
Mar. 2, 1987. .
M. Itoh and H. Ishigaki "Preparation of Superconducting Y-Ba-Cu-O
Thick Film", Japanese Journal of Applied Physics, vol. 27, No. 3,
Mar. 1988, pp. L420-L422. .
Kawasaki et al. "Compositional and Structural Analyses for
Optimizing the Preparation Conductions of Superconducting
(La.sub.1-x Sr.sub.x)CuO.sub.4-8 Films by Sputtering", Japanese
Journal of Applied Physics, Part 2, vol. 26, No. 4, Apr. 1987, pp.
L388-L390. .
Koinuma et al. "Preparation of (La.sub.1-x Sr.sub.x).sub.2
CuO.sub.4-8 Superconducting Films by Screen Printing Method",
Japanese Journal of Applied Physics, Part 2, vol. 26, No. 4, Apr.
1987, pp. L399-L401. .
Meng et al., "High T.sub.c Superconducting Thin Films by Chemical
Spray Deposition", International Journal of Modern Physics B, vol.
1, No. 2 (1987) pp. 579-582. .
Vest et al., "Synthesis of Metallo-organic Compounds for MOD
Powders and Films", MRS Proceedings, Symposium L. Defect Properties
and Processing of High Technology Non-Metallic Materials, Boston,
MA, Dec. 2-4, 1985..
|
Primary Examiner: Ryan; Patrick
Attorney, Agent or Firm: Thomas; Carl O.
Claims
What is claimed is:
1. An element comprised of a substrate and an electrically
conductive layer located on the substrate
characterized in that
the electrically conductive layer is comprised of at least 45
percent by volume of a crystalline rare earth alkaline earth copper
oxide,
the substrate is formed of a material which is capable of
withstanding temperatures necessary to form the conductive layer
and increases the electrical resistance of the conductive layer
when in contact with the rare earth alkaline earth copper oxide
during its crystallization to an electrically conductive form,
a barrier layer having a thickness of greater than 500 Angstroms is
interposed between the electrically conductive layer and the
substrate, the barrier layer being comprised of a metal in its
elemental form or in the form of an oxide or silicide chosen from
the group consisting of (a) group IVa metal or a platinum group
metal and (b) magnesium silicide or silicate.
2. An element according to claim 1 further characterized in that
the conductive layer is restricted to a portion of the substrate
thereby defining a conduction path on the substrate.
3. An element according to claim 2 further characterized in that
the barrier layer is restricted to the same portion of the
substrate as the conductive layer.
4. An element according to claim 1 further characterized in that
the substrate is comprised of glass.
5. An element according to claim 1 further characterized in that
the substrate is comprised of a semiconductor.
6. An element according to claim 5 further characterized in that
the semiconductor substrate is comprised of silicon.
7. An element according to claim 6 further characterized in that
the barrier layer is comprised of a group IVA metal silicide or a
platinum group silicide.
8. An element according to claim 6 further characterized in that
the barrier layer consists essentially of a platinum group
metal.
9. An element according to claim 8 further characterized in that
the barrier layer consists essentially of platinum.
10. An element according to claim 6 further characterized in that
the barrier layer is comprised of magnesium silicate.
11. An element according to claim 1 further characterized in that
the barrier layer is comprised of a group IVA metal in its
elemental form or in the form of an oxide.
12. An element according to claim 11 further characterized in that
the substrate is comprised of a metal oxide other than that
contained in the barrier layer.
13. An element according to claim 12 further characterized in that
the substrate is comprised of alumina.
14. An element according to claim 12 further characterized in that
the barrier layer consists essentially of zirconium or
zirconia.
15. An element according to claim 1 further characterized in that
the conductive layer exhibits a superconducting transition
temperature of at least 30.degree. K.
16. An element according to claim 15 further characterized in that
the conductive layer exhibits a superconducting transition
temperature of at least 80.degree. K.
17. An element according to claim 1 further characterized in that
the conductive layer consists of greater than 70 percent by volume
of a crystalline conductive phase.
18. An element according to claim 1 further characterized in that
the conductive layer is present in the form of a thin film having a
thickness of less than 5 .mu.m.
19. An element according to claim 1 further characterized in that
the conductive layer is present in the form of a thick film having
a thickness of greater than 5 .mu.m.
20. An element according to claim 16 further characterized in that
greater than 45 percent by volume of the conductive layer consists
essentially of a rare earth alkaline earth copper oxide which is in
a tetragonal K.sub.2 NiF.sub.4 crystalline form.
21. An element according to claim 19 further characterized in that
greater than 70 percent by volume of the conductive layer consists
essentially of a rare earth alkaline earth copper oxide which is in
a tetragonal K.sub.2 NiF.sub.4 crystalline form.
22. An element according to claim 21 further characterized in that
the rare earth alkaline earth copper oxide satisfies the metal
ratio:
where
L is lanthanide,
M is alkaline earth metal, and
x is 0.05 to 0.30.
23. An element according to claim 21 further characterized in that
the lanthanide is lanthanum and the alkaline earth metal is barium
or strontium.
24. An element according to claim 23 further characterized in that
x is 0.15 to 0.20.
25. An element according to claim 16 further characterized in that
greater than 45 percent by volume of the conductive layer consists
essentially of a rare earth alkaline earth copper oxide which is in
an R.sub.1 A.sub.2 C.sub.3 crystalline phase.
26. An element according to claim 24 further characterized in that
the rare earth alkaline earth copper oxide consists of yttrium as
the rare earth and barium, optionally in combination with at least
one of strontium and calcium, as the alkaline earth.
27. An element according to claim 26 further characterized in that
the metals present in the R.sub.1 A.sub.2 C.sub.3 crystalline phase
consist essentially of yttrium, alkaline earth, and copper in a
1:2:3 mole ratio.
28. An element according to claim 27 further characterized in that
the conductive layer contains at least one additional phase
comprised of an oxide of at least one of rare earth, alkaline
earth, and copper.
29. An element according to claim 1 further characterized in that
the barrier layer is a thin film having a thickness of less than 5
.mu.m.
30. An element according to claim 1 further characterized in the
barrier layer is a continuous thin film having a thickness of
greater than 1000 .ANG..
31. An element according to claim 29 further characterized in that
the barrier layer is a continuous film having at thickness of
greater than 5000 .ANG..
Description
FIELD OF THE INVENTION
The present invention relates to electrical circuit elements and to
processes for their preparation.
BACKGROUND OF THE INVENTION
The term "superconductivity" is applied to the phenomenon of
immeasurably low electrical resistance exhibited by materials.
Until recently superconductivity had been reproducibly demonstrated
only at temperatures near absolute zero. As a material capable of
exhibiting superconductivity is cooled, a temperature is reached at
which resistivity decreases (conductivity increases) markedly as a
function of further decrease in temperature. This is referred to as
the superconducting transition temperature or, in the context of
superconductivity investigations, simply as the critical
temperature (T.sub.c). T.sub.c provides a conveniently identified
and generally accepted reference point for marking the onset of
superconductivity and providing temperature rankings of
superconductivity in differing materials.
It has been recently recognized that certain rare earth alkaline
earth copper oxides exhibit superconducting transition temperatures
well in excess of the highest previously known metal oxide T.sub.c,
a 13.7.degree. K. T.sub.c reported for lithium titanium oxide.
These rare earth alkaline earth copper oxides also exhibit
superconducting transition temperatures well in excess of the
highest previously accepted reproducible T.sub.c, 23.3.degree. K.
for the metal Nb.sub.3 Ge.
Recent discoveries of higher superconducting transition
temperatures in rare earth alkaline earth copper oxides are
reported in the following publications:
P-1: J. G. Bednorz and K. A. Muller, "Possible High T.sub.c
Superconductivity in the Ba-La-Cu-O System", Z. Phys. B. -Condensed
Matter, Vol. 64, pp. 189-193 (1986) revealed that polycrystalline
compositions of the formula Ba.sub.x La.sub.5-x Cu.sub.5
O.sub.5(3-y), where x=1 and 0.75 and y>O exhibited
superconducting transition temperatures in the 30.degree. K.
range.
P-2: C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, Z. J. Huang, and Y.
Q. Wang, "Evidence for Superconductivity above 40K in the
La-Ba-Cu-O Compound System", Physical Review Letters, Vol. 58, No.
4, pp. 405-407, Jan. 1987, reported increasing T.sub.c to
40.2.degree. K. at a pressure of 13 kbar. At the end of this
article it is stated that M. K. Wu increased T.sub.c to 42.degree.
K. at ambient pressure by replacing Ba with Sr.
P-3: C. W. Chu, P. H. Hor, R. L. Meng, L. Gao, and Z. J. Huang,
"Superconductivity at 52.5K in the Lanthanum-Barium-Copper-Oxide
System", Science Reports, Vol. 235, pp. 567-569, Jan. 1987, a
T.sub.c of 52.5.degree. K. for (La.sub.0.9 Ba.sub.0.1).sub.2
CuO.sub.4-y at high pressures.
P-4: R. J. Cava, R. B. vanDover, B. Batlog, and E. A. Rietman,
"Bulk Superconductivity at 36K in La.sub.1.8 Sr.sub.0.2 CuO.sub.4
", Physical Review Letters, Vol. 58, No. 4, pp. 408-410, Jan. 1987,
reported resistivity and magnetic susceptibility measurements in
La.sub.2-x Sr.sub.x CuO.sub.4, with a T.sub.c at 36.2.degree. K.
when x=0.2.
P-5: J. M. Tarascon, L. H. Greene, W. R. McKinnon, G. W. Hull, and
T. H. Geballe, "Superconductivity at 40K in the Oxygen-Defect
Perovskites La.sub.2-x Sr.sub.x CuO.sub.4-y ", Science Reports,
Vol. 235, pp. 1373-1376, Mar. 13, 1987, reported title compounds
(0.05.ltoreq.x.ltoreq.1.1) with a maximum T.sub.c of 39.3.degree.
K.
P-6: M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Hor, R. L. Meng,
L. Gao, Z. J. Huang, Y. Q. Wang, and C. W. Chu, "Superconductivity
at 93K in a New Mixed-Phase Y-Ba-Cu-O Compound System at Ambient
Pressure", Physical Review Letters, Vol. 58, No. 9, pp. 908-910,
Mar. 2, 1987, reported stable and reproducible superconducting
transition temperatures between 80.degree. and 93.degree. K. at
ambient pressures for materials generically represented by the
formula (L.sub.1-x M.sub.x).sub.a A.sub.b D.sub.y, where L=Y, M=Ba,
A=Cu, D=0, x=0.4, a=2, b=1, and y.ltoreq.4.
The experimental details provided in publications P-1 through P-6
indicate that the rare earth alkaline earth copper oxides prepared
and investigated were in the form of cylindrical pellets produced
by forming an intermediate oxide by firing, grinding or otherwise
pulverizing the intermediate oxide, compressing the particulate
intermediate oxide formed into cylindrical pellets, and then
sintering to produce a polycrystalline pellet. While cylindrical
pellets are convenient articles for cooling and applying resistance
measuring electrodes, both the pellets and their preparation
procedure offer significant disadvantages to producing useful
electrically conductive articles, particularly articles which
exhibit high conductivity below ambient temperature--e.g.,
superconducting articles. First, the step of grinding or
pulverizing the intermediate oxide on a commercial scale prior to
sintering is both time and energy consuming and inherently
susceptible to material degradation due to physical stress on the
material itself, erosion of grinding machinery metal, and handling.
Second, electrically conductive articles rarely take the form of
pellets. Electrically conductive articles commonly include either
thin or thick films forming conductive pathways on substrates, such
as insulative and semiconductive substrates--e.g., printed and
integrated circuits.
CROSS-REFERENCE TO RELATED FILING
Mir, Agostinelli, Peterson, Paz-Pujalt, Higberg, and Rajeswaran
U.S. Ser. No. 046,593, filed May 4, 1987, titled CONDUCTIVE
ARTICLES AND PROCESSES FOR THEIR PREPARATION, commonly assigned,
now issued as U.S. Pat. No. 4,880,770, discloses articles in which
an electrically conductive layer on a substrate exhibits a
superconducting transition temperature in excess of 30.degree. K.
Conductive layers are disclosed comprised of a crystalline rare
earth alkaline earth copper oxide. Processes of preparing these
articles are disclosed in which a mixed metal oxide precursor is
coated in solution and subsequently heated to its thermal
decomposition temperature to create an amorphous mixed metal oxide
layer. The amorphous layer is then heated to its crystallization
temperature. Thin electrically conductive films are formed.
Strom, Carnall, Ferranti, and Mir U.S. Ser. No. 068,391, filed July
1, 1987, titled CONDUCTIVE THICK FILMS AND PROCESS FOR FILM
PREPARATION, commonly assigned, now issued as U.S. Pat. No.
4,908,346, discloses circuit elements comprising an insulative
substrate and means for providing a conductive path between at
least two locations on the substrate including a thick film
conductor which is comprised of a crystalline rare earth alkaline
earth copper oxide layer having a thickness of at least 5 .mu.m.
The thick film conductor is formed by coating a conductor precursor
on the insulative substrate and converting the conductor precursor
to an electrical conductor. The conductor precursor is coated in
the form of particles of metal-ligand compounds of each of rare
earth, alkaline earth, and copper containing at least one thermally
volatilizable ligand. The coated conductor precursor is heated in
the presence of oxygen to form an intermediate coating on the
substrate. The intermediate coating is converted to a crystalline
rare earth alkaline earth copper oxide electrical conductor.
In attempting to form an electrically conductive, particularly
superconductive, rare earth alkaline earth copper oxide layer on a
substrate a difficulty that has been encountered is migration of
substrate and copper containing oxide layer elements upon heating
to the high temperatures required for crystallization, typically in
the range of from 900.degree. to 1100.degree. C. Migration alters
the composition of the copper containing oxide layer and interferes
with formation of the crystal structures required for best
conductivity results. While the difficulty of substrate
contamination of the copper containing oxide layer can be
ameliorated to a degree by increasing its thickness, the choice of
substrates which produce better results in terms of copper
containing oxide layer conductivity has remained restricted,
particularly in forming thin (<5 .mu.m) film thicknesses.
SUMMARY OF THE INVENTION
In one aspect this invention is directed to a circuit element
comprised of a substrate and an electrically conductive layer
located on the substrate. The circuit element is characterized in
that the electrically conductive layer is comprised of a
crystalline rare earth alkaline earth copper oxide, the substrate
is formed of a material which increases the electrical resistance
of the conductive layer when in contact with the rare earth
alkaline earth copper oxide during its crystallization to an
electrically conductive form, and a barrier layer is interposed
between the electrically conductive layer and the substrate. The
barrier layer contains a metal, in its elemental form or in the
form of an oxide or silicide, chosen from the group consisting of
magnesium, a group IVA metal, or a platinum group metal.
In another aspect this invention is directed to a process of
forming a circuit element including coating a conductor precursor
on a substrate and converting the conductor precursor to an
electrical conductor. The process is characterized by the steps of
choosing, as the conductor precursor, metal-ligand compounds of
each of rare earth, alkaline earth, and copper containing at least
one thermally volatilizable ligand; heating the precursor
metal-ligand compounds in the presence of oxygen to produce a
crystalline rare earth alkaline earth copper oxide electrically
conductive layer; choosing as the substrate a material which
increases the electrical resistance of the conductive layer when in
contact with the rare earth alkaline earth copper oxide during its
crystallization to an electrically conductive form; and prior to
coating the conductor precursor on the support forming on the
substrate a barrier layer. The barrier layer contains a metal, in
its elemental form or in the form of an oxide or silicide, chosen
from the group consisting of magnesium, a group IVA metal, or a
platinum group metal.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other advantages of the invention can be better
appreciated by reference to the following detailed description of
preferred embodiments considered in conjunction with the drawings,
in which
FIG. 1 is a schematic diagram showing process steps and articles
produced thereby;
FIG. 2 is a schematic diagram of a portion of a preferred thin film
process;
FIG. 3 is a schematic diagram of an arrangement for coating an
elongated flexible substrate;
FIG. 4 is a schematic diagram of a pattern producing sequence of
process steps; and
FIG. 5 is a schematic diagram of a portion of a preferred thick
film process.
DESCRIPTION PREFERRED EMBODIMENTS
The present invention has as its purpose to make available
electrical circuit elements containing a conductive rare earth
alkaline earth copper oxide layer which is protected from substrate
degradation of its conductive properties by a barrier layer
interposed between the conductive layer and the substrate.
The barrier layer contains a metal, in its elemental form or in the
form of an oxide or silicide, chosen from the group consisting of
magnesium, a group IVA metal, and a platinum group metal. The term
"group IVA metal" refers to the metals titanium, zirconium, and
halfnium occupying Group IVA (International Union of Pure and
Applied Chemistry version) of the periodic table of elements. The
term "platinum group metal" refers to a metal from the second and
third triads of Group VIIIA of the period table--i.e., ruthenium,
rhodium, or palladium forming the second triad or osmium, iridium,
or platinum forming the third triad.
The term "rare earth alkaline earth copper oxide" refers to a
composition of matter containing at least one rare earth element,
at least one alkaline earth element, copper, and oxygen. The term
"rare earth" is employed to designate yttrium and
lanthanides--i.e., elements of the lanthanide series. Lanthanum,
samarium, europium, gadolinium, dysprosium, holmium, erbium, and
ytterbium are particularly preferred lanthanides. The term
"alkaline earth" indicates elements of Group 2 of the Periodic
Table of elements as adopted by the American Chemical Society.
Calcium, strontium and barium are preferred alkaline earth elements
for the practice of this invention.
In keeping with the established practice in the ceramics art of
shortening lengthy chemical names of mixed metal oxides by
substituting acronyms based on the first letters of the metals
present, the term "RAC" is hereinafter employed to indicate
generically rare earth alkaline earth copper oxides. When it is
intended to designate specifically a lanthanide or yttrium as the
rare earth component, L or Y, respectively, is substituted for R;
and when it is intended to designate specifically strontium or
barium as the alkaline earth component, S or B, respectively, is
substituted for A.
A preferred process for preparing an electrical circuit element
according to the present invention is schematically illustrated in
FIG. 1. In Step A of the preparation process, onto a substrate is
coated a solution consisting essentially of a volatilizable film
forming solvent and a barrier metal-ligand compound in addition to
the barrier metal one or more ligands, each of which is thermally
volatilizable. The resulting coated article as schematically shown
consists of substrate 3 and a layer 5 formed by a barrier precursor
(a barrier metal-ligand compound) and a film forming solvent.
In Step B article 1 is heated to a temperature sufficient to
volatilize the ligands and the film forming solvent. The barrier
clad substrate 7 resulting consists of substrate 3 and a barrier
layer 9.
In Step C of the preparation process, onto the barrier layer is
coated a composition consisting essentially of RAC precursors
(metal-ligand compounds of each of rare earth, alkaline earth, and
copper) containing at least one thermally volatilizable ligand. The
resulting coated article 11 as schematically shown consists of the
substrate 3, barrier layer 9, and a layer 13 formed of RAC
precursors.
In Step D of the preparation process, the RAC precursor layer is
converted into an electrically conductive crystalline RAC layer.
Step D entails one or more heating steps in which volatilizable
ligands contained within the RAC precursor are removed from the
layer 13, oxidation of the rare earth, alkaline earth, and copper
metals occurs, and crystallization of the resulting RAC layer
occurs. As schematically shown, product circuit element 15 consists
of the substrate 3, barrier layer 9, and conductive RAC layer 17.
Depending upon specific choices of materials and preparation
techniques, the article 17 can exhibit a high superconducting
transition temperature, herein employed to designate a T.sub.c of
greater than 30.degree. C.
A preferred process for preparing thin (<5 .mu.m) film circuit
elements according to this invention once a substrate having a
barrier layer has been produced can be appreciated by reference to
FIG. 2. In Step C1 of the preparation process, onto a barrier clad
substrate is coated a solution consisting essentially of a
volatilizable film forming solvent and metal-ligand compounds of
each of rare earth, alkaline earth, and copper containing at least
one thermally volatilizable ligand. The resulting coated article
11a as schematically shown consists of barrier clad substrate 7 and
a layer 13a formed by RAC precursors (metal-ligand compounds) and
film forming solvent.
In Step D1 article 11a is heated to a temperature sufficient to
volatilize the ligands and the film forming solvent. The element
15a resulting consists of barrier clad substrate 7 and amorphous
RAC layer 17a. In its amorphous form the RAC coating exhibits
relatively low levels of electrical conductivity.
To convert the amorphous RAc layer to a more highly conductive form
it is necessary to induce crystallization of the RAC layer. In Step
D2 the article 15a is heated to a temperature sufficient to convert
the amorphous RAC layer to a more electrically conductive
crystalline form. In article 15b the RAC layer 17b on barrier clad
substrate 7 is crystalline.
Crystallization of the RAC layer occurs in two stages--crystal
nucleation and crystal growth. It is in some instances preferred to
achieve crystal nucleation at a somewhat different temperature than
is employed for crystal growth. Microscopic examination of articles
at an early stage of crystallization reveals crystal nuclei
surrounded by at least one other RAC phase. Further heating of the
RAC layer at the temperature of nucleation or, preferably, at a
somewhat higher temperature increases the size of the crystal
nuclei at the expense of the surrounding RAC phase or phases until
facets of adjacent crystals are grown into electrically conductive
juxtaposition.
According to accepted percolation theory, for a layer consisting of
conducting spheres randomly located in a surrounding nonconducting
medium the spheres must account for at least 45 percent by volume
of the layer for satisfactory electrical conductivity to be
realized. If conducting particles of other geometric forms,
particularly elongated forms, are substituted for the spheres, the
conducting particles can account for less of the layer volume while
still realizing satisfactory layer electrical conductivity.
Similarly, electrical conductivity can be realized with a lesser
proportion of conducting particles when the surrounding medium is
also conductive. Thus, all layers containing at least 45 percent by
volume electrically conductive particles are by theory electrically
conductive.
Although satisfactory electrical conductivity can be realized with
a lesser volume of the crystalline phase, it is generally
contemplated that in the crystallized RAC layer the crystalline
phase will account for at least 45 percent by volume and preferably
70 percent by volume of the total RAC layer. From microscopic
examination of highly crystalline RAC layers exhibiting high levels
of electrical conductivity it has been observed that layers can be
formed in which little, if any, of the RAC phase surrounding the
crystal nuclei remains. In other words greater than 90 percent (and
in many instances greater than 99 percent) by volume of the RAC
layer is accounted for by the desired crystalline phase.
To achieve crystallization the RAC layer can be heated to any
convenient temperature level. While the barrier layer allows
heating to higher crystallization temperatures than would otherwise
be acceptable, it is generally preferred that the RAC layer be
heated no higher than is required for satisfactory crystallization.
Heating to achieve crystallization can, for example, be limited to
temperatures below the melting point of the RAC composition forming
the layer. Microscopic examination of coatings of some RAC
compositions has revealed that extending heating temperatures or
times beyond those producing crystallization can result in rounding
of crystal corners and edges. It is believed that the rounding
resulting from further heating reduces the area of contact between
adjacent crystal facets and thus restricts the conduction path
through the crystalline RAC layer. From microscopic examination of
RAC layers optimum heating times can be selected for maximizing
both the proportion of the RAC layer accounted for by the
crystalline phase and the desired configuration of the crystals
produced, thereby maximizing electrical conductivity.
Step D3 entails controlled cooling of the RAC layer from its
crystallization temperature. By slowing the rate of cooling of the
crystalline RAC layer imperfections in the crystal lattices can be
reduced and electrical conductivity, which is favored with
increasing order in the crystal structure, is increased. Cooling
rates of 25.degree. C. per minute or less are contemplated until
the crystalline RAC layer reaches a temperature of at least
500.degree. C. or, preferably, 200.degree. C. Below these
temperatures the lattice is sufficiently rigid that the desired
crystal structure is well established. The article 15c produced is
formed of the annealed crystalline RAC layer 17c on barrier clad
substrate 7.
While the article 15c exhibits high levels of electrical
conductivity, in some instances further heating of the article 15c
in an oxygen enriched atmosphere has been observed to increase
electrical conductivity further. In addition to oxygen supplied
from the ligands the oxygen forming the crystalline RAC layer is
obtained from the ambient atmosphere, typically air. It is believed
that in some instances, depending upon the crystal structure being
produced, ambient air does not provide the proportion of oxygen
needed to satisfy entirely the available crystal lattice sites.
Therefore, optional Step D4 entails heating the article 15c in an
oxygen enriched atmosphere, preferably pure oxygen. The object is
to equilibrate the RAC crystalline layer with the oxygen enriched
atmosphere, thereby introducing sufficient oxygen into the crystal
lattice structure. Temperatures for oxygen enrichment of the
crystalline RAC layer are above the minimum 200.degree. C.
annealing temperatures employed in Step D3 described above. To be
effective in introducing oxygen into the crystal lattice
temperatures above those at which the lattice becomes rigid are
necessary. The duration and temperature of heating are
interrelated, with higher temperatures allowing shorter oxygen
enrichment times to be employed. For maximum oxygen enrichment of
the RAC layer the rate of cooling should be less than 25.degree. C.
per minute, preferably less than 15.degree. C. per minute, within
the temperature range of from about 500.degree. C. to 300.degree.
C.
In preparing RAC layers shown to be benefitted by oxygen enrichment
of the ambient atmosphere Step D4 can be consolidated with either
or both of Steps D2 and D3. Oxygen enrichment is particularly
compatible with Step D3, allowing annealing out of crystal lattice
defects and correction of crystal lattice oxygen deficiencies to
proceed concurrently.
The final electrically conductive article 15d is comprised of a
crystalline, electrically conductive RAC layer 17d on barrier clad
substrate 7.
The process described for preparing electrically conductive
articles having RAC layers offers several distinct advantages. One
of the most significant advantages is that the electrically
conductive RAC layer is protected from direct contact with the
substrate throughout the process. This allows a broader range of
substrate materials to be employed and allows better electrical
conduction properties to be achieved. It further allows, but does
not require, higher temperatures to be employed in producing the
conductive RAC layer. Still further, thinner RAC layers having
acceptable electrical conduction properties can be realized. In
many instances the presence of the barrier layer allows
superconductive and particularly high transition temperature
superconductive RAC layer characteristics to be obtained which
would be difficult or impossible to realize in the absence of the
barrier layer.
Another significant advantage of the process described above is
that the proportions of rare earth, alkaline earth, and copper
elements in the final RAC layer 17d exactly correspond to those
present in the RAC precursor layer 13a. In other words, the final
proportion of rare earth, alkaline earth, and copper elements is
determined merely by mixing in the desired proportions in the film
forming solvent the metal-ligand compounds employed as starting
materials. This avoids what can be tedious and extended trial and
error adjustments of proportions required by commonly employed
metal oxide deposition techniques, such as sputtering and vacuum
vapor deposition. Further, the present process does not require any
reduction of atmospheric pressures, and thus no equipment for
producing either high or low vacuum.
A further significant advantage of the process of this invention is
that it can be applied to the fabrication of electrically
conductive articles of varied geometry, particularly those
geometrical forms of electrical conductors most commonly
employed.
The present invention lends itself readily to the preparation of
elongated electrically conductive articles, particularly flexible
elongated electrically conductive articles, such as those employed
as electrical leads, conductive windings in electromagnets,
conductive armature and/or field windings in electrical motors and
generators, conductive windings in transformers, conductive
windings in solenoids, and as long distance electrical transmission
lines. Contemplated flexible elongated electrically conductive
articles include those referred to in the art as rods, wires,
fibers, filaments, threads, strands, and the like. In addition
conductive cladding of ribbons, sheets, foils, and films is
contemplated.
A coating process particularly adapted to coating flexible
substrates can be illustrated by reference to FIG. 3, wherein an
elongated flexible barrier clad substrate 25 is unwound from a
supply spool 27 and passed downwardly over a guide roller 29 into a
reservoir 31. The reservoir contains a film forming solvent with
metal-ligand compounds dissolved therein, as described above in
connection with Step C1, shown as a liquid body 33. The flexible
substrate is drawn over a lower guide roller 35 while immersed in
the liquid and then passed upwardly to a third guide roller 37.
As the flexible substrate is drawn upwardly it emerges from the
liquid body bearing an annular thin, uniform surface layer
corresponding to layer 13a in FIG. 2. Between the reservoir and the
third guide roller the coated substrate is drawn through a heating
zone to complete in different regions of the heating zone process
Steps D1, D2, D3, and D4 sequentially, as previously described. To
accommodate needs for different residence times within the various
heating regions the lengths of the different regions can be
adjusted. Additionally, residence time of a substrate within any
heating region can be further increased by employing laterally
diverting guides, so one or a number of coated substrate
festoon-like path diversions are created within the heating
region.
After passing over the third guide roller the substrate, bearing an
annular crystalline electrically conductive RAC layer is wound onto
a storage spool 39. Where the RAC layer is coated on a flexible
substrate, it is preferred to maintain the thickness of the RAC
layer at 2 .mu.m or less, preferably 1.0 .mu.m or less, so that it
exhibits adequate flexibility. Flexing of the RAC layer required by
guiding and spooling by can be reduced by increasing the radius of
curvature imposed by the third guide roller and storage spool.
The arrangement shown in FIG. 3 for applying a flexible RAC layer
to a flexible substrate is, of course, merely illustrative of a
number of approaches which can be employed to apply a RAC layer to
a flexible substrate. Where it is more convenient to perform
process steps D1, D2, D3, and D4 in a horizontally offset rather
than vertically offset spatial relationship, instead of applying
the RAC precursors and film forming solvent by immersion of the
substrate, other conventional coating approaches can be employed
for application, such as roll coating, spraying, brushing, curtain
coating, extrusion, or the like. It is generally preferred to avoid
guide contact of the coated substrate between application of the
RAC precursors and completion of Step D1. However, once a solid RAC
layer exists on the substrate, guide contact with the substrate
within or between any one of process Step D2, D3, and D4 locations
can be undertaken, as desired for convenient spatial orientation.
Although the process described in connection with FIG. 3 begins
with an elongated flexible substrate coated with a barrier layer,
it is appreciated that essentially similar process steps can, if
desired, be undertaken to form the barrier layer on the substrate
before undertaking formation of the RAC layer.
While flexible electrical conductors of extended length serve a
variety of important applications, there are many other
applications for electrical conductors, particularly those located
on limited portions of substantially planar surfaces of substrates.
Such applications include those served by conventional printed,
integrated, and hybrid circuits. In such circuits limited, if any,
flexibility of the electrical conductor is required, but an ability
to define areally--i.e., pattern, the electrical conductor with a
high degree of precision is in many instances of the utmost
importance. The present invention is compatible with precise
patterning of the electrical conductor on a substrate surface.
Patterning of an electrical conductor according to this invention
is illustrated by reference to FIG. 4. Barrier clad substrate 7 is
coated on its upper planar surface with a uniform RAC precursor
layer 13a as described above in connection with process Step C1 to
form initial coated article 11a. Process Step D1, described above,
is performed on article 11a to produce article 15a, described
above, comprised of amorphous RAC layer 17a and barrier clad
substrate 7.
The amorphous RAC layer lends itself to precise pattern definition
and produces results generally superior to those achieved by
patterning the RAC precursor layer from which it is formed or the
crystalline RAC layer which is produced by further processing. The
RAC precursor layer is often liquid before performing process Step
D1 and is in all instances softer and more easily damaged in
handling than the amorphous RAC layer. The crystalline RAC layer
cannot be etched with the same boundary precision as the amorphous
RAC layer, since etch rates vary from point to point based on local
variations in the crystal faces and boundaries presented to the
etchant. Patterning of either the RAC precursor layer or the
crystalline RAC layer is specifically recognized as a viable
alternative to patterning the amorphous RAC layer for applications
permitting more tolerance of conductor dimensions. For example,
screen printing the RAC precursor layer on a substrate to form a
printed circuit is specifically contemplated.
While the amorphous RAC layer can be patterned employing any
conventional approach for patterning metal oxides, for more precise
edge definitions the preferred approach is to photopattern the
amorphous RAC layer employing any of the photoresist compositions
conventionally employed for the precise definition of printed
circuit or integrated circuit conductive layers. In a preferred
form of the process, a uniform photoresist layer 23 is applied to
the amorphous RAC layer 17a as indicated by process Step D5. The
photoresist layer can be formed by applying a liquid photoresist
composition to the amorphous RAC layer, spinning the substrate to
insure uniformity of the coating, and drying the photoresist.
Another approach is to laminate a preformed photoresist layer
supported on a transparent film to the amorphous RAC layer.
The photoresist layer is then imagewise exposed to radiation,
usually through a mask. The photoresist can then be removed
selectively as a function of exposure by development. Positive
working photoresists are removed on development from areas which
are exposed to imaging radiation while negative working
photoresists are removed only in areas which are not exposed to
imaging radiation. Exposure and development are indicated by
process Step D6. Following this step patterned photoresist layer
23a is left on a portion or portions of the amorphous RAC layer
17a. Although the patterned residual photoresist layer is for
convenience shown of a simple geometrical form, it is appreciated
that in practice the patterned photoresist can take any of a wide
variety of geometrical forms, including intricate and thin line
width patterns, with line widths ranging into the sub-micrometer
range.
Following patterning of the photoresist layer, portions of the RAC
layer which are not protected by the photoresist can be selectively
removed by etching, as indicated by process Step D7. This converts
the amorphous RAC layer 17a to a patterned RAC layer 17e confined
to areas corresponding to that of the photoresist. Note that in the
process of etching the barrier clad substrate may be modified by
removal of the barrier in unprotected areas to produce a modified
barrier clad substrate 7a. Whether or not the unprotected barrier
is removed will depend, of course, on the specific etchant
employed. However, it is important to note that there is no
requirement that the etchant be selective to the amorphous RAC
layer as opposed to the barrier material.
Following patterning of the amorphous RAC layer the patterned
photoresist is removed, as indicated by process Step D8. The final
article, shown in FIG. 4 as consisting of the partially barrier
clad substrate 7a and patterned amorphous RAC layer 17e, is then
further processed as indicated in FIG. 2, picking up with process
Step D2. The crystalline RAC layer formed in the final product
conforms to the patterned amorphous RAC layer.
In the process of preparing a patterned article described above it
is noted that once an article is formed having an amorphous RAC
layer on a substrate it can be patterned to serve any of a wide
variety of circuit applications, depending upon the circuit pattern
chosen. It is therefore recognized that instead of or as an
alternative to offering patterned articles for sale a manufacturer
can instead elect to sell articles with unpatterned amorphous RAC
layers on a barrier clad substrate, with or without an unpatterned
photoresist layer, to subsequent fabricators. It will often be
convenient in this instance to locate a removable layer or film
over the amorphous RAC layer for its protection prior to further
fabrication. The subsequent fabricator can undertake the patterned
exposure and further processing required to produce a finished
electrical circuit element.
To crystallize a RAC layer and to perform the optional, but
preferred annealing and oxygen enrichment steps both the substrate
and RAC layer are heated uniformly. This can be done employing any
conventional oven. In some instances, however, either to protect
the substrate from rising to the peak temperatures encountered by
the RAC layer or simply to avoid the investment in an oven by
fabricator, it is contemplated that the RAC layer will be
selectively heated. This can be accomplished by employing a radiant
heat source, such as a lamp--e.g., a quartz lamp. Lamps of this
type are commercially available for achieving rapid thermal
annealing of various conventional layers and can be readily applied
to the practice of the invention. These lamps rapidly transmit high
levels of electromagnetic energy to the RAC layer, allowing it to
be brought to its crystallization temperature without placing the
substrate in an oven.
A diverse approach for producing patterned electrical conductors
can be practiced by employing article 15a comprised of the uniform
amorphous RAC layer 17a and barrier clad substrate 7 as a starting
element. Instead of patterning the amorphous RAC layer followed by
crystallization of the remaining portions of the layer, the
amorphous RAC layer is imagewise addressed to produce
crystallization selectively only in areas intended to be rendered
electrically conductive. For example, by addressing the amorphous
RAC layer with a laser, areas directly impinged by the laser beam
can be selectively crystallized to an electrically conductive form,
leaving the remaining amorphous areas unaffected. To define the
conductive pattern generated it is only necessary to control the
path of the laser beam.
Where a manufacturer chooses to sell an article consisting of a
uniform amorphous RAC layer on a barrier clad substrate, this
approach to patterning can be more attractive than the uniform
heating processes described above, since no oven is required to
reach the temperatures typically required for crystallization. The
fabricator choosing laser patterning may, in fact, require no other
heating equipment. Thus, a very simple approach to forming a
crystalline RAC pattern is available.
It is, of course, recognized that additional heating for purposes
of annealing or oxygen saturation can be undertaken, following lamp
or laser crystallization, by heating in any desired manner. One
approach is to heat at least amorphous layer 17a of the article 15a
to a temperature above its minimum annealing temperature and then
laser address the heated article. This facilitates annealing and
oxygen enrichment without requiring heating the entire article
uniformly to the significantly higher levels otherwise required for
crystal nucleation and growth.
Another variation on the laser patterning approach is to follow the
laser responsible for crystallization with one or more passes from
a lower intensity laser beam to retard the rate of cooling and
thereby enhance annealing. For example, a laser beam can be swept
across an area of the substrate surface to produce crystallization
and then reduced in intensity or defocused and swept back across
the same area to facilitate annealing. By defocusing the laser beam
on subsequent passes over the same area the laser energy is spread
over a larger area so that the maximum effective temperature levels
achieved are reduced. The advantage of employing one laser for
multiple passes is that alignments of laser beam paths are more
easily realized. Additionally or alternatively, the rapidity with
which the laser is swept across the exposed area can be adjusted to
control the temperature to which it heats the RAC layer. Other
laser scanning variations are, of course, possible.
Both lamp heating and laser scanning allow a broader range of
substrate materials to be considered, particularly those which,
though capable of withstanding ligand and solvent volatilization
temperatures, are susceptible to degradation at crystallization
temperatures. By choosing wavelengths in spectral regions to which
the amorphous RAC layer is opaque or at least highly absorbing,
direct radiant heating of the substrate can be reduced or
eliminated. In this instance the bulk of the radiation is
intercepted in the RAC layer before it reaches the substrate
barrier. The substrate is also protected from direct radiant
heating by the barrier layer. By proper choice of radiant energy
wavelengths the barrier layer can reflect a high proportion of
total radiant energy received.
To avoid coating imperfections in the thin film process described
above the thickness of an amorphous RAC layer produced in a single
process sequence is maintained at 1 .mu.m or less, preferably 0.6
.mu.m or less, and optimally 0.4 .mu.m or less, a single process
sequence being understood to constitute the steps described above
for forming an amorphous RAC layer. By repeating the process
sequence one or more times an amorphous RAC layer of any desired
thickness can be built up.
In the process of fabrication described above the formation of the
desired RAC layer begins with the formation of a RAC precursor
layer. To form the precursor layer a solution of a film forming
solvent, a rare earth metal compound, an alkaline earth metal
compound, and a copper compound is prepared. Each of the rare
earth, alkaline earth, and copper compounds consists of metal ion
and one or more volatilizable ligands. By "volatilizable" it is
meant that the ligand or its component elements other than oxygen
can be removed from the substrate surface at temperatures below the
crystallization temperature of the RAC layer. In many instances
organic ligands breakdown to inorganic residues, such as
carbonates, at relatively low temperatures, with higher temperature
being required to remove residual carbon. A ligand oxygen atom
bonded directly to a metal is often retained with the metal in the
RAC layer, although other ligand oxygen atoms are generally
removed. At least 95 percent of the ligands and their component
atoms other than oxygen are preferably outgassed at temperatures of
less than 600.degree. C. On the other hand, to avoid loss of
materials before or during initial coating of the metal-ligand
compounds, it is preferred that the ligands exhibit limited, if
any, volatility at ambient temperatures. Metal-ligand compounds
having any significant volatility below their decomposition
temperature are preferably avoided.
Metalorganic compounds, such as metal alkyls, alkoxides,
.beta.-diketone derivatives, and metal salts of organic
acids--e.g., carboxylic acids, constitute preferred metal-ligand
compounds for preparing RAC precursor coatings. The number of
carbon atoms in the organic ligand can vary over a wide range, but
is typically limited to less than 30 carbon atoms to avoid
unnecessarily reducing the proportion of metal ions present.
Carboxylate ligands are particularly advantageous in promoting
metal-ligand solubility. While very simple organic ligands, such as
oxalate and acetate ligands, can be employed in one or more
metal-ligands compounds, depending upon the film forming solvent
and other metal-ligand compound choices, it is generally preferred
to choose organic ligands containing at least 4 carbon atoms. The
reason for this is to avoid crystallization of the metal-ligand
compound and to improve solubility. When heating is begun to remove
the film forming solvent and ligands, the solvent usually readily
evaporates at temperatures well below those required to remove the
ligands. This results in leaving the metal-ligand compounds on the
substrate surface. When the ligands have few carbon atoms or, in
some instances, linear carbon atom chains, crystallization of the
metal-ligand compounds occurs. In extreme cases crystallization is
observed at room temperatures. This works against the molecular
level uniformity of rare earth, alkaline earth, and copper ions
sought by solution coating. Choosing organic ligands exhibiting 4
or more carbon atoms, preferably at least 6 carbon atoms, and,
preferably, ligands containing branched carbon atom chains, reduces
molecular spatial symmetries sufficiently to avoid crystallization.
Optimally organic ligands contain from about 6 to 20 carbon
atoms.
Instead of increasing the molecular bulk or modifying the chain
configuration of organic ligands to avoid any propensity toward
metalorganic compound crystallization on solvent removal, another
technique which can be employed is to incorporate in the film
forming solvent a separate compound to act as a film promoting
agent, such as a higher molecular weight branched chain organic
compound. This can, for example, take the form of a branched chain
hydrocarbon or substituted hydrocarbon, such as a terpene having
from about 10 to 30 carbon atoms.
The film forming solvents can be chosen from a wide range of
volatilizable liquids. The primary function of the solvent is to
provide a liquid phase permitting molecular level intermixing of
the metalorganic compounds chosen. The liquid is also chosen for
its ability to cover the substrate uniformly. Thus, an optimum film
forming solvent selection is in part determined by the substrate
chosen. Generally more desirable film forming properties are
observed with more viscous solvents and those which more readily
wet the substrate alone, or with an incorporated wetting agent,
such as a surfactant, present.
It is appreciated that a wide variety of ligands, film promoting
agents, and film forming solvents are available and can be
collectively present in a virtually limitless array of composition
choices.
Exemplary preferred organic ligands for metal organic compounds
include metal 2-ethylhexanoates, naphthenates, neodecanoates,
butoxides, isopropoxides, rosinates (e.g., abietates),
cyclohexanebutyrates, and acetylacetonates, where the metal can be
any of the rare earth, alkaline earth, or copper elements to be
incorporated in the RAC layer. Exemplary preferred film forming
agents include 2-ethylhexanoic acid, rosin (e.g., abietic acid),
ethyl lactate, 2-ethoxyethyl acetate, and pinene. Exemplary
preferred film forming solvents include toluene, 2-ethylhexanoic
acid, n-butyl acetate, ethyl lactate, propanol, pinene, and mineral
spirits.
As previously noted, the metal-ligand compounds are incorporated in
the film forming solvent in the proportion desired in the final
crystalline RAC layer. The rare earth, alkaline earth, and copper
can each be reacted with the same ligand forming compound or with
different ligand forming compounds. The metal-ligand compounds can
be incorporated in the film forming solvent in any convenient
concentration up to their saturation limit at ambient temperature.
Generally a concentration is chosen which provides the desired
crystalline RAC layer thickness for the process sequence. Where the
geometry of the substrate permits, uniformity and thickness of the
metal-ligand coating can be controlled by spinning the substrate
after coating around an axis normal to the surface of the substrate
which has been coated. A significant advantage of spin coating is
that the thickness of the coating at the conclusion of spinning is
determined by the contact angle and viscosity of the coating
composition and the rate and time of spinning, all of which can be
precisely controlled. Differences in the amount of the coating
composition applied to the substrate are not reflected in the
thickness of the final coating. Centrifugal forces generated by
spinning cause excess material to be rejected peripherally from the
article.
The foregoing process of coating RAC precursors in solution is
particularly suited to forming thin films. The term "thin film" is
employed to indicate films having thicknesses of less than 5 .mu.m,
such films most typically having thicknesses of less than 1 .mu.m.
The term "thick film" is employed in its art recognized usage to
indicate films having thicknesses in excess of 5 .mu.m.
A preferred process for producing thick film electrically
conductive RAC layers on barrier clad substrates can be appreciated
by reference to the schematic diagram shown in FIG. 5. In Step C2 a
composition containing particles of metal-ligand compounds is
obtained. Each particle contains rare earth, alkaline earth, and
copper atoms in the same ratio desired in the final RAC containing
conductive layer. Further, the atoms are intimately intermixed so
that the composition of each particle is preferably essentially
uniform. Associated with the metal atoms and completing the
compounds are volatilizable ligands, which can be all alike or
chosen from among different ligands.
The particles can be of any size convenient for coating. The
particles can exhibit a mean diameter up to the thickness of the
coating to be formed, but more uniform films are realized when the
mean particle diameters are relatively small in relation to the
thickness of the film to be formed. The particles are preferably
less than about 2 .mu.m in mean diameter, optimally less than 1
.mu.m in mean diameter. The minimum means diameter of the particles
is limited only by synthetic convenience.
A preferred technique for producing metal-ligand compound particles
is to dissolve the rare earth, alkaline earth, and copper metal
ligand compounds in a mutual solvent and then to spray the solution
through an atomizing nozzle into a gaseous atmosphere. The solvent
is chosen to be evaporative in the gaseous atmosphere. Thus, the
individual particles are dispersed in the gaseous atmosphere as
liquid particles and eventually come to rest at a collection site
as either entirely solid particles or particles in which the
proportion of solvent has been sufficiently reduced that each of
the metal-ligand compounds present has precipitated to a solid
form. In the latter instance the particles by reason of the
residual solvent, now no longer acting as a solvent, but only as a
continuous dispersing phase, form a paste. The paste constitutes a
highly convenient coating vehicle. When the particles are collected
in a friable form with all or substantially all of the initially
present solvent removed, it is recognized that a paste can still be
formed, if desired, by adding to the particles a small amount of a
liquid to promote particle cohesion--i.e., to constitute a
paste.
Only a very small amount of liquid is required to promote particle
cohesion and thereby form a paste. Typically the liquid constitutes
less than 20 percent of the total composition weight and preferably
less 15 percent of the total compositon weight. While optimum paste
consistencies can vary depending upon the selection of processes
for coating the paste, it is generally contemplated that the paste
viscosity will be in the range of from 5.times.10.sup.4 to
3.times.10.sup.6 centipoise, preferably from 1.times.10.sup.5 to
2.5.times.10.sup.6 centipoise.
While atomization and drying can be undertaken in air at room
temperatures, it is recognized that any gaseous medium which does
not detrimentally react with the metal-ligand compounds can be
employed. Further, the temperature of the liquid forming the
particles or, preferably, the gaseous medium can be increased to
accelerate the solvent evaporation rate, provided only that such
elevated temperatures in all instance be maintained below the
thermal decomposition temperatures of the metal-ligand
compounds.
The advantage of solidifying the metal-ligand compounds while they
are trapped within discrete particles is that bulk separations of
the rare earth, alkaline earth, and copper are prevented. The
particle preparation approach offers distinct advantages over
simply evaporating bulk solutions to dryness in that each particle
produced by the process of this invention contains the desired
ratio of rare earth, alkaline earth, and copper elements. This
produces a solid particle coating composition of microscale
uniformity.
In Step C3 of the preparation process, onto a substrate are coated
the metal-ligand compound particles, preferably combined with a
carrier liquid to form a coatable paste or slurry. The resulting
coated article 11b as schematically shown consists of barrier clad
substrate 7 and a layer 13b formed by RAC precursors (metal-ligand
compounds) and film forming solvent. Although the layer 13b is
shown coextensive with the barrier clad substrate 7, it is
appreciated that the particles are well suited, particularly when
coated in the form of a paste or slurry, to being laid down in any
desired pattern on the barrier clad substrate. The paste can, for
example, be deposited by any of a variety of conventional image
defining coating techniques, such as screen or gravure printing.
Since thick conductive films are most commonly formed in the art by
screen printing, the present invention is highly compatible with
conventional printed circuit preparation processes.
The ligands in the RAC precursor compounds of the thick film
process like those of thin film process form no part of the final
article and therefore can be chosen based solely upon convenience
in performing the process steps described above. Ligands are chosen
for their ability to form solutions in which rare earth, alkaline
earth, and copper combined with the ligands are each soluble in the
desired proportions and to be volatilizable during heating to form
the intermediate RAC layer. Inorganic ligands, such as nitrate,
sulfate, and halide ligands, are illustrative of preferred ligands
satisfying the criteria set forth above. Nitrate, bromide, and
chloride ligands are particularly preferred. In general the ligands
are chosen so that each of the rare earth, alkaline earth, and
copper ligand compounds exhibit approximately similar solubility
characteristics.
Any evaporative solvent for the metal-ligand compounds can be
employed for particle fabrication. Again, the solvent forms no part
of the final article. Polar solvents, such as water or alcohols
(e.g., methanol, ethanol, propanol, etc.), are particularly suited
for use with metal-ligand compounds containing the inorganic
ligands noted above.
Where a paste is coated, the paste contains either a small residual
portion of the original solvent for the metal-ligand compounds or a
different liquid to promote cohesion. The liquid fraction of the
paste must be volatilizable. The evaporative solvents noted above
all satisfy this criteria.
The paste, apart from the metal-ligand particles, can be identical
in composition to conventional inks employed in screen printing.
Screen printing inks normally contain an active ingredient (in this
instance supplied by the metal-ligand particles), binders to
promote substrate adhesion (such as glass frit or crystalline oxide
powder), screening agents used to enhance the rheological
properties of the ink--usually a higher molecular weight polymer,
such as poly(vinyl alcohol) or poly(ethylene glycol), and a liquid,
most commonly water or an alcohol. It is a particular advantage of
this invention that the metal-ligand particles and liquid together
provide excellent rheological and adhesion properties without the
necessity of incorporating other screen printing ink
ingredients.
Heating step D can be performed as described above can then be
undertaken to produce final article 15e consisting of thick film
RAC layer 17f on barrier clad substrate 7 as described above in
connection with FIG. 1. The overall heating step D can include the
same sequence of steps D1, D2, D3, and D4 described above in
connection with FIG. 2.
In addition to all of the advantages described above for the
preferred thin film forming process, a particular advantage of
thick film process is that it readily lends itself to the formation
of electrical conductor patterns on limited portions of
substantially planar substrate surfaces without resorting to
uniform coatings followed by etching to define a pattern. This is a
convenience which assumes an added level of importance in producing
thick film conductors. Thus, the present process is readily applied
to the fabrication of printed and hybrid circuits. The thick film
process can also be employed to form RAC layers of less than 5
.mu.m in thickness--that is, it is capable of forming either thick
or thin film electrical circuit elements.
To achieve articles according to this invention which are not only
electrically conductive, but also exhibit high T.sub.c levels,
thereby rendering them attractive for high conductivity (e.g.,
superconducting) electrical applications, RAC layers are produced
in specific crystalline forms. One specifically preferred class of
high T.sub.c articles according to this invention are those in
which the crystalline RAC layer consists of greater than 45 percent
by volume of a rare earth alkaline earth copper oxide which is in a
tetragonal K.sub.2 NiF.sub.4 crystalline phase. The K.sub.2
NiF.sub.4 crystalline phase preferably constitutes at least 70
percent and optimally at least 90 percent by volume of the RAC
layer.
A preferred rare earth alkaline earth copper oxide exhibiting this
crystalline phase satisfies the metal ratio:
where
L is lanthanide,
M is alkaline earth metal, and
x is 0.05 to 0.30.
Among the preferred lanthanides, indicated above, lanthanum has
been particularly investigated and found to have desirable
properties. Preferred alkaline earth metals are barium and
strontium. Optimum results have been observed when x is 0.15 to
0.20.
Thus, in specifically preferred forms of the invention LBC or LSC
layers exhibiting a tetragonal K.sub.2 NiF.sub.4 crystalline phase
are present and capable of serving high conductivity applications,
including those requiring high T.sub.c levels and those requiring
superconductivity at temperatures in excess of 10.degree.K.
Specific LBC layers in the tetragonal K.sub.2 NiF.sub.4 crystalline
phase have been observed to have T.sub.c levels in excess of
40.degree. K.
Another specifically preferred class of high T.sub.c articles
according to this invention are those in which the crystalline RAC
layer consists of greater than 45 percent by volume of a rare earth
alkaline earth copper oxide which an R.sub.1 A.sub.2 C.sub.3
crystalline phase, believed to be an orthorhombic Pmm2 or
orthorhombically distorted perovskite crystal phase. This phase
preferably constitutes at least 70 percent by volume of the RAC
layer.
A preferred rare earth alkaline earth copper oxide exhibiting this
crystalline phase satisfies the metal ratio:
where
M is barium, optionally in combination with one or both of
strontium and calcium.
Although the R.sub.1 A.sub.2 C.sub.3 crystalline phase by its
crystal lattice requirements permits only a specific ratio of
metals to be present, in practice differing ratios of yttrium, rare
earth, and copper are permissible. The metal in excess of that
required for the R.sub.1 A.sub.2 C.sub.3 crystalline phase is
excluded from that phase, but remains in the YAC layer.
Processing temperatures employed in forming the amorphous RAC
layers and in subsequently converting the amorphous layers to
crystalline layers can vary significantly, depending upon the
specific RAC composition and crystal form under consideration.
Crystallization is in all instances achieved below the melting
point of the RAC composition. Melting points for RAC compositions
vary, but are typically well above 1000.degree. C. Typical RAC
crystallization temperatures are in the range of from about 900 to
1100.degree. C. Where crystal nucleation and growth are undertaken
in separate steps, nucleation is preferably undertaken at a
somewhat lower temperature than crystal growth.
In some instances X-ray diffraction has revealed the presence of
microcrystals in the amorphous RAC layer, although limited to minor
amounts. While crystallization of the metal-ligand compounds, which
tends to separate the metals into different phases, is generally
avoided, crystallization which occurs during or immediately
following ligand volatilization is not objectionable, since metals
absent their ligands are free to form mixed metal oxides.
A preferred technique for producing a high T.sub.c coating
employing an amorphous layer of the LAC composition metal ratio I
above, particularly an LBC or LSC composition, is to heat the
amorphous layer on the substrate to a temperature of about
925.degree. to 975.degree. C. to achieve crystal nucleation.
Crystal growth is then undertaken at a temperature of about
975.degree. to 1050 .degree. C. Following conversion of the LAC
layer to the tetragonal K.sub.2 NiF.sub.4 crystalline phase, it is
cooled slowly at rate of of 25.degree. C. or less per minute until
it reaches a temperature of 550.degree. to 450.degree. C. The LAC
layer is then held at this temperature or reheated to this
temperature in the presence of an oxygen atmosphere until oxygen
equilibration is substantially complete, typically about 20 to 120
minutes.
A preferred technique for producing a high T.sub.c coating
employing an amorphous layer of the YAC composition satisfying
metal ratio II above, particularly YBC, is to heat the amorphous
layer on the substrate to a temperature of a temperature greater
than 900.degree. C., but less than 950.degree. C., optimally
920.degree. to 930.degree. C. Following conversion of the LAC layer
to the R.sub.1 A.sub.2 C.sub.3 crystalline phase, it is cooled
slowly at rate of of 25.degree. C. or less per minute until it
reaches a temperature of 750.degree. to 400.degree. C. The YAC
layer is then held at this temperature or reheated to this
temperature following cooling in the presence of an oxygen
atmosphere until oxygen equilibration is substantially complete,
typically about 20 to 120 minutes.
In general any conventional electrical conductor substrate, whether
it is itself conductive, insulative, or semiconductive, capable of
withstanding processing temperatures can be employed. For example,
substrates in the form of metal wires, glass fibers, ceramic and
glass plates, semiconductor wafers, and the like all possess
sufficient thermal stability to be employed as substrates in the
circuit elements of this invention.
Because temperatures in the range of 900.degree. C. and higher are
required for RAC layer crystallization to its preferred
electrically conductive forms, all substrate materials examined
have been observed to interact to some degree with the RAC layer to
degrade its electrical conduction characteristics--that is, to
increase. This includes common circuit element substrate materials
such as silicon (e.g., polycrystalline and monocrystalline silicon
of the type employed in semiconductor manufacture), silicon dioxide
(e.g., fused, crystalline, and amorphous forms), silicon nitride
(e.g., nitride layers grown on monocrystalline silicon), and
alumina (e.g., amorphous, polycrystalline, and monocrystalline
forms). Slightly more unusual substrate materials, such as alkaline
earth oxides (e.g., amorphous or monocrystalline magnesia and
monocrystalline strontium titanate), chosen specifically for their
compatibility with the RAC layer have also been observed to degrade
the conductivity characteristics of crystalline RAC layers when the
substrate is formed in direct contact with the RAC layer. This is
particularly true in forming thin films as described above,
although substrate interactions can be reduced or controlled by
undertaking repetitions of the RAC layer preparation process
through the step of forming the amorphous RAC layer prior to
crystallization, as discussed above.
It is the discovery of this invention that specifically selected
metals as well as their oxides and silicides when interposed
between a substrate (specifically illustrated by, but not limited
to the common substrates identified above) and the RAC layer
enhances the electrical conduction properties of the RAC layer.
These barrier materials minimize diffusion or migration between the
substrate and the RAC layer during the heating stages of its
formation, particularly the crystallization stage, which requires
temperatures in the range of 900.degree. C. and higher.
While thick films of barrier material can separate the RAC layer
and the substrate, it is a significant advantage of this invention
that thin film barrier layers are effective. Thin films minimize
the amount of material required to form barrier layers and are much
more compatible with the microminiaturization requirements of
electronic components, particularly integrated circuit components,
where both precise pattern definitions, more readily generated with
very thin films, and limited disparities in layer heights,
achievable only with very thin films, are often required.
Observable improvements in RAC layer conduction properties can be
realized with extremely thin barrier layers. For example, at least
some reduction in substrate interaction of the RAC layer can be
expected so long as the barrier material forms a continuous layer.
Continuous barrier layer thicknesses of greater than 500,
preferably greater than 1000, and optimally greater than 5000
Angstroms are contemplated.
The effectiveness of the barrier layer at a particular thickness is
related to its crystallinity. An ideal barrier layer would be a
continuous monocrystalline layer, such as a layer epitaxially grown
on an underlying substrate. Any such requirement would, however,
greatly restrict the classes of substrates capable of being
modified by a barrier layer. Microcrystalline barrier layers are
specifically contemplated. Any diffusion within the the barrier
layer predominantly directed to circuitous intersticial boundaries
between adjacent microcrystals. The microcrystals themselves act as
barriers to diffusion.
Preferred barrier layers are amorphous. Achieving amorphous
deposition places no restriction on substrate selection. In other
words any type of substrate described above can be employed in
combination with an amorphous barrier layer. At the same time,
there are no grain boundaries present to provide preferential
internal diffusion paths. Hence migration through amorphous barrier
layers is highly impeded.
Amorphous barrier layers can be readily formed by employing the
techniques described above for forming amorphous RAC layers,
particularly those described for forming thin films. That is,
barrier layers can be formed starting with barrier precursors,
barrier metal-ligand compounds, where the ligands are chosen in the
same manner as described in connection with RAC precursors.
Since the barrier layer is present during the heating steps which
produce the crystalline RAC layer, it is appreciated that the
barrier layer initially produced as an amorphous layer and lying
beneath an amorphous RAC layer may be converted to a
microcrystalline form in the course of producing a crystalline RAC
layer. Using mixtures of ligands to form the barrier metal-ligand
compounds, employing mixtures of barrier metals or other compatible
metals, or both, can be relied upon to increase the complexity of
atomic spatial relationships within the barrier layer and thereby
favor an amorphous as opposed to a crystalline physical form.
Barriers of magnesia and oxides of group IVA metals (particularly
zirconia) are most conveniently formed by the use of barrier
metal-ligand compounds, as described above. Suitable barrier
metal-ligand compounds can be formed by substitution of the barrier
metal for any one of the rare earth, alkaline earth, and copper in
the metal-ligand compounds described above for forming the RAC
layer.
A preferred approach for forming elemental metal barrier layers is
to deposit the metal on the substrate by conventional electron beam
deposition techniques. In subsequent heating, preferably before
deposition of the RAC precursor coating, the barrier metal can, if
desired, be converted to the corresponding oxide or silicide. For
example, where the substrate receiving the barrier metal is
silicon, the formation of a silicide can be readily achieved.
Platinum group metals are contemplated for deposition in elemental
form. When coated on a silicon substrate platinum group metals can
be converted to the corresponding silicide. The platinum group
metals are particularly suited for forming silicides, since they
are essentially resistant to oxygen attack. Restriction of the
availability of oxygen can be used to cause the remaining barrier
metals to favor silicide formation.
Zirconium when coated on a silicon substrate in the presence of
oxygen has been observed to form mixtures of zirconia and zirconium
silicide. Magnesium when coated on a silicon substrate and heated
in oxygen forms magnesium silicates.
The times and temperatures effective to convert a barrier metal to
the corresponding oxide or silicide can be detemined by routine
investigation. For heating times of from 10 to 60 minutes in oxygen
temperatures in the range of from 600.degree. to 1500.degree. C.
are effective, the specific choice of conditions depending upon the
exact choice of barrier metal, substrate and the degree of reaction
by the barrier metal being sought.
It is appreciated that the barrier layer can alternatively be
formed by any other convenient conventional preparation process. It
is specifically contemplated to form barrier layers by sputtering,
vacuum vapor deposition, and metal-organic chemical vapor
deposition procedures.
EXAMPLES
Details of the preparation and performance of articles according to
this invention are illustrated by the following examples:
EXAMPLE 1
A thin film of zirconia (ZrO.sub.2) was produced on a polished
fused quartz (glass) substrate. The film was prepared by the
thermal decomposition of a precursor solution consisting of toluene
as a solvent and 50 percent by weight, based on total weight, of
zirconium n-propoxide tri-neodecanoate. The zirconium n-propoxide
tri-neodecanoate was prepared by mixing stoichiometric amounts of
zirconium n-propoxide and neodecanoic acid at room temperature.
The above precursor was spin coated onto the fused quartz substrate
at 5000 rpm over a period of 20 seconds. The film was then heated
to 500.degree. C. on a hot plate. The resulting zirconia film was
specularly transparent and approximately 1400 .ANG. in thickness.
This coating technique was performed a total of four times,
resulting in a final zirconia barrier layer thickness of 5600
.ANG..
A high transition temperature superconductive YBC layer was
deposited onto the barrier layer using the following technique:
A yttrium containing solution was prepared by mixing and reacting
yttrium acetate with a stoichiometric excess of 2-ethylhexanoic
acid to produce yttrium tri(2-ethylhexanoate) in 2-ethylhexanoic
acid. The resulting solution contained 7.01 percent by weight
yttrium, based on total weight.
A copper containing solution was prepared by mixing and reacting
copper acetate with a stoichiometric excess of 2-ethylhexanoic acid
to form copper di(2-ethylhexanoate). This solution contained 6.36
percent by weight copper, based on total weight.
A 0.81 g sample of the yttrium containing solution and a 1.92 gram
sample of the copper containing solution were mixed followed by the
addition of 0.66 gram of barium di(cyclohexanebutyrate), 0.4 gram
of toluene, and 0.7 gram of rosin. Heat was applied until all
ingredients had entered solution, thereby forming a YBC precursor
solution.
The YBC precursor solution was deposited onto the zirconia barrier
layer by spin coating at 5000 rpm for 20 seconds. The coated
substrate had a smooth and uniform appearance with no imperfections
being noted on visual inspection, indicating favorable rheological
properties.
The YBC precursor coated barrier layer and substrate were heated on
a hot plate to 550.degree. C. to eliminate organic ligands from the
coating. The resulting film was 4000 .ANG. in thickness. The film
forming process was twice repeated.
The amorphous RAC layer exhibited a 1:2:3 atomic ratio of Y:Ba:Cu
and a thickness of 1.2 .mu.m. The amorphous YBC layer was heated to
900.degree. C. for 3 minutes and allowed to cool at a rate of less
than 25.degree. C. per minute.
A second element was prepared identically as described, except that
the zirconia barrier layer was omitted. Both elements produced were
examined by X-ray diffraction. The element lacking the zirconia
barrier layer showed no trace of an orthorhomic perovskite
structure in the YBC layer, whereas a well defined orthorhombic
perovskite structure was observed in the YBC layer formed on the
zirconia barrier layer. This demonstrated that the Y.sub.1 B.sub.2
C.sub.3 crystalline form necessary for superconductivity was
produced in the element containing a zirconia barrier layer while
no such superconductive crystal structure was achieved in the
absence of the zirconia barrier layer.
EXAMPLE 2
Example 1 was repeated, but with a monocrystalline silicon
substrate substituted for fused quartz and with the crystallization
temperature of 900.degree. C. being applied for 5 minutes instead
of 3 minutes.
Similar results were observed. The element incorporating the
zirconia barrier layer exhibited an orthorhombic perovskite crystal
structure in the LBC layer while the element lacking the zirconia
barrier layer did not.
EXAMPLE 3
Example 2 was repeated, except that the zirconia layer produced was
only 1400 .ANG.in thickness. The results were identical to those
reported in Example 2.
EXAMPLE 4
Example 1 was repeated, excepted that the substrate used was made
of sapphire cut in the (1102) orientation. Also the number of YBC
layers was increased from 3 to 4.
Although the YBC film deposited directly on sapphire showed some
indications of a perovskite structure, the YBC deposited over a
zirconia barrier layer exhibited a much better defined perovskite
structure.
At room temperature resistance of the YBC film deposited on the
zirconia barrier layer was approximately 1 order of magnitude lower
than the corresponding YBC film deposited directly on the
substrate.
EXAMPLE 5
Example 4 was repeated, except that polycrystalline alumina was
substituted for sapphire as the substrate material. Similar results
were observed. The element with the zirconia barrier layer
interposed between the substrate and the YBC film exhibited a more
clearly defined perovskite crystal structure than was the case with
the barrier layer omitted.
EXAMPLE 6
A silicon wafer was coated with a 2 .mu.m thick layer of titanium
metal by electron beam deposition. Next YBC precursor solution of
the following composition was spin coated onto the barrier layer
coated substrate:
30.4 g 2-Ethylhexanoic acid,
4.089 g Yttrium tri(2-ethylhexanoate),
8.125 g Barium di(cyclohexane butyrate),
8.26 g Copper di(2-ethylhexanoate),
4.0 g Toluene, and
7.6 g Rosin.
The YBC precursor solution was prepared by dissolving the yttrium,
barium, and copper carboxylates and rosing in the 2-ethylhexanoic
acid and toluene solvent mixture. The solution was refluxed for 5
minutes, allowed to cool to room temperature, and then filter using
a 1.2 .mu.m filter.
A coating of the YBC precursor solution was then produced on the
titanium barrier layer of the silicon wafer by spinning the silicon
wafer at 2000 rpm for 20 seconds. The YBC precursor coating was
then heated in air to 650.degree. C. in a Fischer.RTM. Model 495
ashing furnace to volatilize the organic ligands. The element was
held at this temperature for 5 minutes to produce an amorphous YBC
layer. This coating procedure was performed 8 times in
sequence.
The amorphous YBC layer exhibited a 1:2:3 atomic ratio of Y:Ba:Cu.
The amorphous YBC layer was converted to an electrically conductive
crystalline form by heating in air to 875.degree. C. in the ashing
furnace. The sample was held at this temperature for 3 minutes and
then allowed to cool slowly at a rate of 6.degree. C. per
minute.
X-ray diffraction analysis of the conductive crystalline YBC layer
confirmed that it exhibited a well defined orthorhombic perovskite
structure. A minor amount of a second, copper oxide phase was also
present.
When the example was repeated, but with the titanium barrier layer
omitted, no orthorhombic perovskite structure was detected by X-ray
diffraction analysis.
EXAMPLE 7
Example 6 was repeated, except that prior to coating the YBC
precursor solution on the barrier layer the barrier layer and
substrate were heated to 1000.degree. C. in oxygen and held at this
temperature for 30 minutes. X-ray diffraction analysis of the
barrier layer indicated that it contained a mixture of TiO.sub.2
and TiO. The electrically conductive crystalline YBC layer was
similar to that produced by Example 6.
EXAMPLE 8
Example 6 was repeated, except that zirconium was electron beam
deposited in place of titanium. The conductive crystalline YBC
layer produced on the zirconium barrier layer appeared on analysis
by X-ray diffraction similar to that produced in Example 6.
EXAMPLE 9
Example 8 was repeated, except that prior to coating the YBC
precursor solution on the barrier layer the barrier layer and
substrate were heated to 1000.degree. C. in oxygen and held at this
temperature for 30 minutes. X-ray diffraction analysis of the
barrier layer indicated that it contained a mixture of zirconia
(ZrO.sub.2) and zirconium silicide (ZrSi.sub.2). The electrically
conductive crystalline YBC layer was similar to that produced by
Example 6.
EXAMPLES 10 and 11
These examples were identical to Examples 8 and 9, respectively,
except that the thickness of the zirconium deposited was reduced to
2500 .ANG.. The electrically conductive crystalline YBC layers were
similar to those reported in Examples 7 and 8.
EXAMPLES 12 and 13
These examples were performed identically as examples 10 and 11,
except that the thickness of the zirconium layer deposited was
reduced to 500 .ANG.. X-ray diffraction indicated the major portion
of the crystals in the YBC coating to consist of copper and yttrium
oxides in an equal atomic ratio with a minor portion of the
crystals being cuprous oxide. Thus, under these preparation
conditions the thickness of the zirconium deposited was not
sufficient to produce the preferred orthorhombic perovskite
crystalline structure.
EXAMPLE 14
Example 7 was repeated, except that a magnesium layer of 1 .mu.m in
thickness was formed by vacuum vapor deposition. X-ray diffraction
indicated that heating in oxygen converted the barrier layer to
magnesium silicate (Mg.sub.2 SiO.sub.4) and magnesia (MgO). X-ray
diffraction analysis of the electrically conductive crystalline YBC
layer revealed an orthorhombic perovskite crystal structure.
EXAMPLE 15
Example 6 was repeated, except that a platinum layer of 2500 .ANG.
in thickness was formed by electron beam deposition. X-ray
diffraction analysis of the electrically conductive crystlline YBC
layer revealed an orthorhombic perovskite crystal structure to
account for the major phase, with cuprous oxide (CuO) and Cu.sub.2
Y.sub.2 O.sub.5 accounting for minor phases.
While the invention has been described in terms of one barrier
layer being interposed between the RAC layer and the substrate, it
is appreciated that two or more of the barrier layers described
above can be employed in combination between the substrate and the
RAC layer.
The invention has been described in detail with particular
reference to preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
* * * * *